| The micron-size micro-space existed on pollution insulator by the long term accumulation of dust particles makes a possible of discharge propagating on the surface of pollution layer or inside the layer with the structure changing sometimes. However the influence of salt density, dust density, the local conductivity and the atmospheric environment on the pollution flashover has been extensively studied, the fact that the flashover has two modes of discharge is neglected. One mode is surface discharge along the pollution layer and the another one is discharge in the pollution layer which is surface discharge in two-phase mixture(TMP) and influenced by the micro-structure. Furthermore, it lack of researching on the thermal characteristics of pollution flashover and the influence on the flashover for the micro-discharge.The experiments of surface discharge in TMP and the numerical simulations are the main work of this paper, and the results obtained are as follows:The simulation pollution solution was brushed quantitatively on the surface of a dielectric plate and it was pollution layer. And the thermal characteristic of pollution surface discharge under the condition of the surface electric field with weak vertical component was studied and the thermodynamics mechanism of pollution flashover with three stages was put forward. At the first stage of no discharge, the heat originated from the surface leakage current for the pollution layer. At the second stage of surface discharge of slipping, the heat were generated by the leakage current and the discharge current simultaneously. In the final stage of flashover, the heat was mainly produced by the big discharge current. Among the surface discharge stage, the leakage current was the main heat source, and the surface temperature caused was the second item expressions of the applied voltage and higher than that caused by surcharge discharge. The heat on the layer surface would transfer into the volume with the increase of contamination degree, so that the flashover voltage would decrease. When the flashover occurred, the heat in the volume would transfer into the surface conversely. Meanwhile, the floating angle of arcing decreased with the contamination degree, the decrease of heat in the volume and the increase of heat on the surface.The phenomena of surface discharge in two-phase mixture on the condition of electric field with strong vertical component was studied on the pollution layer of the accumulation of sand particles. It was that the discharge in a layer was found. It could be confirmed that the flashover progress was the problem of surface discharge in two-phase mixture by the micro-structural features R3/H. R is the radius of sand particle and H is the thickness of sand layer. The discharge in a layer was initial on the junction of the sand layer, the electrodes and the insulation dielectric, and propagated in the thin sand layer near the disc outside edge. It was found the flashover voltage was influenced by the thickness of sand layer, marked as H. When H was smaller than0.5mm, the flashover voltage was lower than that without sand in the open air. Oppositely, when H was bigger than1mm, the flashover voltage increased with H and it was ever higher than that without sand. Then the micro-structure of sand layer and the mechanism of discharge in sand layer could be described by the parameter of R3/H, Which would change the distribution of potential, current and electric field. Only when the value of H was up, the surface electric field near the high-voltage electrode would decrease but it would increase near the outside edge of sand layer. It would be helpful to explain that the influence of the thickness of sand layer on the flashover voltage and the location of flashover occurred.The tube discharge and the discharge of two-phase in the confined space with a tube were studied and the thermal temperature on dielectric was measured. There were foul discharge modes among three stages. The first stage was the initialization of corona and dielectric barrier discharge(DBD) with a little temperature rising of less than10℃near the high-voltage electrode. Then the second stage was slipping discharge along the tube wall from corona and DBD. The location of the highest temperature was near the high-voltage electrode and it would move towards the grounding electrode as the applied voltage increased. The final stage was the breakdown of spark. The temperature of tube wall would be up to200℃and distribute uniformly. The experiments of DC breakdown of tube was confirmed that the breakdown voltage was higher than that in the open air on the condition of small tube and big gas gap, for the wall charge and the leakage current through tube wall. The experiments indicated that the critical size of λ was approximately2.5mm that is the transverse estimated size of electron avalanche of streamer in air at atmospheric pressure.The surface discharge with pollution layer and particle layer were numerical simulated respectively. And it was concluded that the wall charge and the surface conductivity were the critical factors. When the permittivity of layer surface was big, the surface discharge would be promoted and accelerated. The electric field would be enhanced by the initial wall charge. Similarly, the wall charge of sand particle had an important influence on the mode of surface discharge. Because of the strong field in the micro-gap between the charged particles and the second electron emission of particle, micro-discharge occurred. Finally it propagated into the discharge through the layer.The tube discharge was numerical simulated. The influence of the heat accumulated and the the initial charge accumulated in the tube wall on tube discharge was studied together. The strong surface field for the accumulated wall charge was the initial factor of surface discharge occurred. Meanwhile, the heat released by repeated discharge was convective and it would promote the progress of thermal ionization and maintain the surface discharge propagate along the tube wall stable. |
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